Heat exchanger



FIG. 1 is an isometric view of a front, top, and right side of an ornamental design for a heat exchanger;

FIG. 2 is a front elevational view of the heat exchanger of FIG. 1;

FIG. 3 is a rear elevational view of the heat exchanger of FIG. 1;

FIG. 4 is a left side elevational view of the heat exchanger of FIG. 1, wherein the right side elevational view is a mirror image of the left side elevational view;

FIG. 5 is a top plan view of the heat exchanger of FIG. 1; and,

FIG. 6 is a bottom plan view of the heat exchanger of FIG. 1.

The broken lines are for the purpose of illustrating environmental structure and form no part of the claimed design. 

The ornamental design for a heat exchanger, as shown and described. 